This isn't possible without stopping MySQL for some seconds. You must mount your RAID device somewhere, stop MySQL, copy /var/lib/mysql to the new device, rename /var/lib/mysql to something else (like /var/Lib/mysql_old), create a new and empty /var/Lib/mysql directory, remount the RAID device to /var/lib/mysql, chown /var/lib/mysql to mysql:mysql, and start MySQL.